Digital closed loop proportional hydraulic pressure controller

ABSTRACT

A digitally controlled current to pressure converter (CPC) and method of controlling same is provided. The method of controlling includes the step of periodically imparting symmetrically-opposed movement of a control valve of the CPC to loosen and flush accumulated silt therefrom. More particularly, the method may include the step of periodically introducing a small-amplitude symmetrically-opposed impulse to a controller that actuates a hydraulic control shaft of a three-way rotary valve. Also provided is a method of preventing malfunction due to faulty input or feedback signals received by the CPC, and a method of detecting the health status of multiple CPCs when used in a redundant configuration.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This patent application is a Divisional of co-pending U.S. patentapplication Ser. No. 13/489,832, filed Jun. 6, 2012, which is aContinuation of U.S. patent application Ser. No. 12/024,148, filed Feb.1, 2008, now U.S. Pat. No. 8,215,329, the entire teachings anddisclosures of which are incorporated herein by reference thereto.

FIELD OF THE INVENTION

The present invention relates generally to positioning control systemsfor steam and fuel valves and their associated servo mechanisms, andmore particularly to current to pressure converters (CPC) that convertan analog current control signal to hydraulic pressure for usetherewith.

BACKGROUND OF THE INVENTION

Many control components in plants, buildings, and other manufacturingfacilities utilize hydraulic pressure to position the actuators, controlvalves, or operating surfaces of these components. Such componentsinclude steam control valves, fuel valves, dampers, vanes, etc. Onecommon means of positioning the actuators is to provide a linearlyincreasing variable hydraulic pressure that acts upon the piston of alinear hydraulic actuator or vane of a rotary cylinder. The opposingforce required to counterbalance this variable pressure and thus createproportionality can be in the form of an opposing spring, or hydraulicpressure.

While purely hydraulic valving and control systems have been utilized toeffectuate the positioning of these control components, modernelectronic controls have increased the functionality and flexibility ofthe system control. Such component, system, and plant controllerstypically utilize PLC- or DCS based computing systems to monitor andcontrol the various components within the system. The use of suchcontrollers, therefore, necessitates the use of an interface componentthat is capable of taking the control signal outputs from suchcontrollers and converting those electronic control signals intohydraulic control signals that can effectuate the positioning andcontrol of the hydraulic actuated components. One such interface controldevice is known as a current to pressure converter (CPC).

A typical CPC is configured to receive an analog 4-20 mA control signalfrom a system or plant controller. This 4-20 mA control signal is thenproportionally converted into a hydraulic output pressure by the CPC. Assuch, the CPC may be thought of as a electrohydraulic, pressureregulating valve. Such CPCs typically include an internal 3-way valve,actuator, pressure sensor or pressure feedback mechanism, and on-boardanalog electronics. A cascade control loop is typically employed toachieve closed loop control of pressure. The first control loop comparesthe input control signal or pressure setpoint to the measured feedback.The difference is then modified by a circuit or algorithm to generate aposition demand signal which is the input of the second control loop.The position demand signal is then compared to the measured position andthe difference modified by a circuit or control algorithm to produce adrive signal which will open or close the actuator to match the positiondemand over time. The combined operation of the dual control loops inconjunction with the actuator and valve ensures that the measuredfeedback matches the setpoint over time.

The valve internal to the CPC is a three way control valve. At thecenter position, the control port is isolated from both the supply anddrain. By moving the valve slightly above the center position, thecontrol port is connected to the supply port resulting in an increase inpressure. By moving the valve below the center position, the controlport is connected to the drain, resulting in a decrease in pressure. Areturn spring is provided in the assembly such that in the event of lossof power or an electric fault, the valve will move to the “minimumpressure” position which in most applications is the direction to shutdown the turbine.

While current CPC's perform adequately in many applications, theaccuracy of such control in some installations may be adversely affectedby the thermal drift associated with the analog control circuits withinthe CPC itself. Further, CPC malfunction has been noted in some systemsthat do not typically change the positioning of the control componentfor long periods of time, or in backup CPC's in systems that utilize aprimary and backup regulator to ensure system operation in case ofmalfunction of the primary CPC. Such malfunctions have been determinedto be caused by the build-up of silt and other contaminates that haveaccumulated on the valve element during a long period of stagnantcontrol.

In view of the above, the inventor has recognized a need for a new andimproved CPC that overcomes the inaccuracies resulting from thermaldrift of the analog control circuits and that ensures continuedoperation even after extended periods of inactivity that would otherwiseresult in silt build-up on the valving element leading to malfunction.Embodiments of the present invention provide such a new and improvedCPC.

BRIEF SUMMARY OF THE INVENTION

In view of the above, embodiments of the present invention provide a newand improved current to pressure converter (CPC) that overcomes one ormore of the problems existing in the art. More particularly, embodimentsof the present invention provide a new and improved CPC that does notsuffer from inaccuracies resulting from thermal drift of the analogcontrol components used in some CPC's. Still further, embodiments of thepresent invention may also eliminate or greatly reduce the likelihood ofCPC malfunction in installations experiencing long periods of inactivitybetween repositioning of the control valve therein.

An embodiment to the present invention includes fully digital processingof the control loop and diagnostic signals, which beneficially reducesthe thermal drift associated with the prior analog control systems usedto control the CPC. An onboard pressure sensor is also incorporated inone embodiment to provide closed loop control of the output pressure.Such onboard pressure sensor offers improved linearity and accuracy overprevious CPC's that utilized force feedback devices.

Improved reliability is provided in one embodiment by including aredundant, dynamic sealing system with an intermediate passage to thehydraulic drain circuit to ensure that the pressure drop across theoutboard seal is very low, thereby minimizing the potential for leakageand improving the reliability of the CPC. One embodiment of the presentinvention also includes provisions for improved redundancy and faultmanagement to ensure failsafe operation in the event of internalcomponent failure.

Reliable operation is also provided in embodiments of the presentinvention through the inclusion of an anti-silting algorithm that willdeter the accumulation of fine silting particles. Such accumulation hasbeen problematic and a chronic problem on steam turbines which use theturbine's lube/oil for the hydraulic supply. Embodiments of thisalgorithm will introduce a small amplitude, symmetrically opposed,impulse on the position of the rotating valve. This impulse will rotatethe valve element very slightly to loosen and flush away any silt thathas accumulated on the valve element. In one embodiment, the impulse isof a very short duration, and includes opposed negative, then positive,components. In such an embodiment the result is a near net zerodisplacement of fluid in the output circuit controlled by the CPC. Assuch, there is no or only minimal detectable behavior of the outputservo during the anti-silting impulse. Such small amplitude,symmetrically opposed impulse may be applied periodically, at fixed timeintervals, and can be easily adjusted by the user based on the oilquality of the application.

In other embodiments of the present invention, the digital controllermay monitor driver current levels, and may increase or decrease theinterval between impulses automatically upon the detection of a variancein driver current levels that may indicate the buildup of contamination,or lack thereof, to effectuate a self tuning of the interval based uponactual need.

Reliable operation is also provided in embodiments of the presentinvention through the inclusion of redundant control inputs for eitherthe main control setpoint or the pressure transducer used for closedloop control. Historically operation of the turbine is often adverselyimpacted by failure of the main controller, wiring between thecontroller and CPC, or the transducer used for pressure feedback. In thepreferred embodiment a second input is provided which can be configuredto monitor a 2^(nd) controller, or receive a 2^(nd) command signal viaan independent wiring path from the turbine controller, or a 2^(nd)pressure feedback transducer. As such, the user can configure theinstallation of the CPC for additional robustness to these failuremodes, and the logic executed within the CPC will utilize the 2^(nd)input signal to maintain operation in the event of failure.

In applications requiring the highest level of reliability, two CPC'sare sometimes applied in a tandem arrangement. In this configuration,failure modes of the turbine control system, the control wiring betweenthe control and the CPC, or failure of the CPC itself can largely bemitigated. In the preferred embodiment the two CPC's have a status linkwired directly from one unit to the other. As such, each CPC knows theoperational status of the other and should a fault occur within the CPCin control of the system, the back-up unit can resume control in anextremely short time interval without intervention from the main turbineor plant controller. This minimizes the potential for dynamictransitions which could adversely affect the speed or load of theturbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a simplified system level diagram of a typical installation ofan embodiment of a CPC constructed in accordance with the teachings ofthe present invention;

FIG. 2 is a cross-sectional illustration of an embodiment of a CPCconstructed in accordance with the present invention;

FIG. 3 is a hydraulic schematic of the CPC of FIG. 2;

FIG. 4 is a simplified CPC controller block diagram illustrating oneembodiment of a controlled constructed in accordance with the teachingsof the present invention;

FIG. 5 is simplified system level diagram of a master/slave CPCinstallation constructed in accordance with the teachings of the presentinvention;

FIG. 6 is a simplified block diagram illustrating the redundancy switchover logic utilized in an embodiment to the present invention forredundant control inputs;

FIG. 7 is a simplified block diagram illustrating the redundancy switchover logic utilized in an embodiment to the present invention forredundant transducer feedback;

FIG. 8 is a graphical illustration of the output pressure verses commandinput available from one embodiment of the present invention;

FIG. 9 is a graphical illustration of the symmetrical anti-siltingimpulse function effectuated by one embodiment to the present invention;and

FIG. 10 is a graphical illustration of a single symmetrical anti-siltingimpulse event utilized in one embodiment of the present invention.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, and specifically to FIG. 1, there isillustrated a typical turbine control system 100 to which embodiments ofthe present invention are particularly well suited. However, while thefollowing description will utilize this exemplarity installation of anembodiment of a CPC 102 constructed in accordance with the teachings ofthe present invention, this installation is not meant to be limiting,but will aid in the understanding of the functionality and advantagesprovided by such a CPC 102. Other installations and operations ofembodiments of the present invention will be recognized by those skilledin the art from the following description, and application thereof arespecifically reserved.

In such an installation as illustrated in FIG. 1, the CPC 102 mayinterface with a turbine controller 104, such as a Model 505/505E SteamTurbine Digital Controller available from the assignee of the instantinvention. Such a turbine controller 104 is typicallymicroprocessor-based and is designed to operate steam turbines 106, suchas, e.g. single extraction and/or admission steam turbines. Embodimentsof the CPC 102 may also interface with a system plant controller 108 toprovide, for example, feedback information from the CPC 102 as well asfault information.

In operation, the CPC 102 receives command signals from the turbinecontroller 104 in the form of an analog control signal varying between 4and 20 mA. The control logic within the CPC 102 processes this controlcommand signal and either increases or decreases the hydraulic pressureto the turbine's servo system 110. The servo system 110 is operable tovary a steam control valve 112 to vary the operating speed of the steamturbine 106. In the system illustrated in FIG. 1, the steam turbine 106is used to drive a load, such as a pump 114, a generator (not shown),etc. As will be discussed more fully below, when the CPC 102 determinesthat additional hydraulic pressure is needed, it sources hydraulic fluidfrom tank 116 via pump 118 and filter 120 to the servo system 110. Whenthe CPC 102 determines that less hydraulic pressure is needed, the CPCoperates to drain hydraulic fluid back to tank 116.

To effectuate such operational control, the CPC 102 includes digitalcontrol mounted internal to the housing 122 on a digital electronicassembly (referred to hereinafter as a digital printed circuit board(PCB) 124) as may be seen in FIG. 2. This digital PCB 124 is protectedby a PCB cover 126 and may be accessed by removing the access cover 128which meets with the housing 122 to form a sealed enclosure.

The controller mounted on this digital PCB 124 controls the position ofthe hydraulic control shaft 130 via a rotary limited angle torque (LAT)actuator 132. Specifically, the LAT 132 includes a permanent magnetrotor 134 that is directly coupled to the hydraulic control shaft 130.The position of the rotor 134 is measured by a solid state integratedcircuit on the digital PCB 124 which detects the direction of thesensing magnet 136 on the hydraulic control shaft 130. The H-bridgedrive of the LAT 132 is regulated by the microprocessor on the digitalPCB 124 to control the position of the hydraulic control shaft preciselyto maintain the pressure set point received from the turbine controller104.

The hydraulic control shaft 130 rotates within a hydraulic controlbushing 138 that is ported to form a three-way rotary valve 140. Thisthree-way rotary valve 140 controls the hydraulic fluid flow from thesupply (not shown) to the control port 142 and from the control port 142to the drain (not shown). In a preferred embodiment, both the hydrauliccontrol shaft 130 and the hydraulic control bushing 138 are made ofstainless steel. This offers precise, reliable, andcontamination-tolerant operation on typical oils used for steam turbinelubrication.

To provide failsafe operation in the event of component or powerfailure, a spiral power spring 144 operates the bottom portion of thehydraulic control shaft 130 in the lower cavity 146 of the housing 122.Access to the spiral power spring 144 is via lower cover 148. In theevent of power failure, the spiral power spring 144 will providesufficient rotary power to rotate the hydraulic control shaft 130 into afailsafe condition. One embodiment of this failsafe condition couplesthe control port 142 with the drain.

To protect the dry stator 150 a redundant dynamic sealing system 152 isutilized. This redundant dynamic sealing system 152 includes anintermediate passage 154 to the hydraulic drain circuit. This ensuresthat the pressure drop across the outboard seal 156 is very low,minimizing the potential for leakage and improving the reliability ofthe CPC 102.

Precise hydraulic pressure control is aided by the inclusion of apressure transducer 158 that provides the microprocessor with a preciseindication of the current hydraulic pressure supplied via control port142. This on-board pressure transducer 148 improves the linearity andaccuracy of the closed loop control of the output pressure over priorCPC's that utilized a force feedback device.

The simplified hydraulic schematic of FIG. 3 provides an illustration ofthe operational connection of the hydraulic control circuit within theCPC 102. As may be seen, the digital PCB 124 provides a position controlsignal to the LAT 132 to position the three-way rotary valve 140.Position feedback for the closed loop control is provided by the sensingmagnet 136. As discussed briefly above, the 3-way rotary valve 140 isdesigned to couple the control port 142 with either the supply port 160or the drain port 162 to control the pressure, sensed by pressuretransducer 158, to the servo system 110 in accordance with the controlsignal received from the turbine controller 104 (see FIG. 1).

As may be seen from this hydraulic schematic of FIG. 3, when the 3-wayrotary valve 140 is in the mid position, the control port 142 is coupledto neither the supply port 160 nor the drain port 162. In thisconfiguration, the output hydraulic pressure is held constant at controlport 142. If the pressure drops below the set point pressure, the PCB124 will command the LAT 132 to rotate the valve such that control port142 is connected to the supply port 160 to increase the pressure in thecontrol port 142 (as sensed by pressure transducer 158). If, however thehydraulic pressure at the control port 142 is higher than desired, thecontrol circuitry on the PCB 124 will command the LAT 132 to positionthe 3-way rotary valve 140 so as to couple the control port 142 to thedrain port 162 to lower the pressure at the control port 142.

This dynamic pressure control is controlled by a digital controlalgorithm 164 executed within the digital PCB 124, such as thatillustrated in simplified block diagram form in FIG. 4, to whichreference is now made. As may be seen from the simplified block diagram,the digital controller 164 may include a control demand input 166 and,in one embodiment, a redundant control demand input 168. This or each ofthese inputs are passed through signal conditioners 170. The controllogic for dealing with two controlled demand inputs will be discussedmore fully below. Once this control demand input is determined, it isused in a pressure proportional integral derivative (PID) control loop172. As illustrated, this PID control loop 172 receives a control oilpressure feedback from pressure transducer 158 and a feedback positionsignal from the sensing magnet 136 (not shown). Based on thisinformation, the digital controller 164 controls the position of thevalve 140 via the LAT 132.

The digital controller 164 also includes in an embodiment a service port174 that interfaces with the CPC supervisory logic 176 via a serviceport communications module 178. This service port allows, for example,field programming and diagnostics via a PC or microprocessor-basedservice tool. The CPC supervisory logic 176 monitors the operation ofthe CPC and includes outputs for a shutdown relay 180, an alarm relay orred unit status 182, a master slave indication 184 where suchfunctionality is provided (see description below regarding FIG. 5),and/or analog output 186 that is capable of driving, for example, acontrol pressure meter 188.

In the embodiments of the CPC 102 of the present invention that areutilized in a master/slave environment such as that shown in FIG. 5, theCPC supervisory logic 176 also includes master/slave inputs 190. Thesemaster/slave inputs 190 may be utilized with external jumpers or relaysto establish an initial role for the particular CPC based oninstallation location. However, even in systems that utilize initialrole designations, the CPC supervisory logic 176 includes logic tofacilitate control transfer between CPCs upon the detection of a faultof a master or other conditions that may necessitate or make desirablesuch control transfer.

As illustrated in FIG. 4, the master indication output 184 includes twostatus lines that connect to the other CPC(s). The CPC supervisory logic176 in each CPC 102 monitors these lines. If either CPC 102 fails, or ifthe signal line fails, the other CPC will take control within aprescribed period of time, i.e. 10 milliseconds. This minimizes anytransition bump to the servo system 110 which can occur with prior CPC'swherein such delay between switching to a new CPC control upon thefailure of the master may be 100 milliseconds or more depending upon thecomputing rate of the main controller. As will be recognized by thoseskilled in the art, such a long transition time may result in a verylarge control pressure transient and corresponding changes in speed orload of the controlled plant.

A simplified single line illustration of a system 100′ utilizing a slaveCPC 102A and associated slave turbine controller 104A in addition to themaster CPC 102B and associated master turbine controller 104B is shownin FIG. 5. As discussed above, control of the hydraulic pressure to theservo system 110 is provided by the master CPC 102B until a fault isdetected therewith. Upon determination that the master CPC 102B hasfailed, the select valve 210 switches hydraulic pressure control to theslave CPC 102A. In an embodiment of the present invention, the two CPCs102A/102B have a status link 212 wired directly from the masterindication circuit 184 (see FIG. 4) of one CPC 102A to the other CPC102B. As such, each CPC 102A/102B knows the operational status of theother CPC 102B/102A. Should a fault occur within the master CPC 102B incontrol of the system 100′, the back-up unit or slave CPC 102A canassume control in an extremely short time interval without interventionfrom the main turbine or plant controller. This minimizes the potentialfor dynamic transitions which could adversely affect the speed or loadof the turbine 106.

FIG. 6 illustrates a functional block diagram of the redundancy switchover logic 196 utilized in embodiments of the present invention thatinclude multiple control inputs 166, 168. Specifically, each controlinput 166, 168 is monitored by input diagnostics 192, 194 to evaluationthe reasonableness of the values on each of the two control inputs 166,168. If the input diagnostics 192, 194 determines that one of the inputsignals 166, 168 is considered faulty, e.g., out of range, unstable,etc., it will be voted out of the chain by the redundancy switch overlogic 196 and the CPC 102 will continue to operate only based on thenon-faulty input. If the redundancy switch over logic 196 determinesthat both signals are faulty, then the CPC is commanded to a failsafeposition and an appropriate alarm or other indication is provided. Ifboth input signals are considered valid, although different, theredundancy switch over logic 196 may simply select one of the two inputsfor control, may average the two inputs, take the higher, the lower, orother input logic processing to provide the operating setpoint signalfor the CPC 102.

Similar redundancy switch over logic 198 may be utilized along withfeedback signal diagnostics 200, 202 to evaluate the reasonableness ofmultiple feedback signals 204, 206 in embodiments that utilize multiplefeedbacks, e.g., multiple feedback transducers, position sensors, etc.This feedback redundancy switch over logic 198 is illustrated in FIG. 7.

As illustrated in FIG. 8, the digital controller 164 provides multipleadjustments that greatly increase the type of installations andfunctionality within each installation of the CPC 102 of the presentinvention. As shown in the output pressure versus command input scalinggraphical illustration of FIG. 8, an adjustment can be made of theminimum pressure level and the maximum pressure level. The minimumpressure level adjustment sets the level of output pressure. Adjustingthe minimum pressure level changes all points of the output pressureuniformly. That is, adjusting the minimum level sets the minimum travelof the servo system 110 (see FIG. 1). The maximum pressure leveladjustment sets the maximum output pressure when the output commandcontrol signal is at 20 milliamps. Increasing this level increases theslope of the line 208 and the position of the output servo system foreach value of the input signal.

The PID control loop 172 settings may also be adjusted to tune thedynamic performance of the CPC 102. The proportional gain may beadjusted to set the amount of gain (proportional action). In oneembodiment ten percent gain is used. As will be recognized by thoseskilled in the art, a high gain provides a fast response time, but cancause instability. The integral gain may also be adjusted to set thestability (integration action) of the PID control loop 172. Thisstability cooperates with the proportional gain setting to providestable operation. Finally, a derivative component of the PID controlloop 172 may also be adjusted to set the amplitude of the output dither.

As discussed above, failures of CPC's in installations that utilizeredundant or backup CPC's or in systems that do not vary the hydraulicoutput for extended periods of time have been determined to be a resultof the accumulation of fine silting particles. These failures areparticularly acute in steam turbine applications such as that shown inFIG. 1 wherein the turbine's lubricating oil is used as the hydrauliccontrol fluid. To overcome this problem, an embodiment of the CPC 102 ofthe present invention include a symmetrical anti-silting impulsefunction as part of its control logic. Specifically, the digitalcontroller 164 includes an algorithm which introduces a small amplitude,symmetrically opposed, impulse on the position of the three-way rotaryvalve 140. This small impulse will cause a rotation of the three-wayrotary valve 140 very slightly in both directions.

As illustrated in FIG. 9, these impulses may occur at fixed or periodictimes. The selection of the interval may be set or varied based upon theparticular installation and the amount of silt and contaminationtypically associated therein. These small impulses are effective inloosening and flushing away any silt that has accumulated on the valveelement during the period of inactivity. The plot of FIG. 9 shows animpulse interval set at five hours. However, this interval may bemanually or automatically varied within the CPC 102.

In one embodiment the automatic variation of the anti-silting impulse isbased upon a detection of a deviation in the driver current levelsneeded to effectuate movement of the three-way rotary valve 140.Increased driver current requirements are an indication of the build-upof contamination on the valve. When such a condition is detected, thefrequency of anti-silting impulses may be increased. Similarly, if thedriver current is not sensed as being at a level that might indicatecontamination build-up on the valving element, the anti-silting intervalmay be extended so as to prolong the life of the internal bearings andseals.

As illustrated in FIG. 10, each anti-silting impulse event is of veryshort duration and very low amplitude. Indeed, the use of the digitalcontroller 164 allows adjustment of various parameters controlling theanti-silting impulse, such as the amplitude, frequency, and durationthereof. With regard to the amplitude of the anti-silting impulse,typically a one percent impulse is sufficient to flush contaminants fromthe CPC 102. However, this may be adjusted as required to achieve theanti-silting benefit within the parameters of the system performance. Inone embodiment amplitudes up to five percent can be set either at thefactory or in the field via the service tool interface discussed above.The frequency of the anti-silting impulse may also be adjusted asdiscussed above. While FIG. 9 illustrated an interval of five hours,other embodiments may utilize the impulse only once per day, althoughother installations may require impulses generated at a frequency from,for example, one minute to three months. This interval may also be setat the factory or in the field. Finally, the duration of eachanti-silting impulse may also be varied. Depending on overall systemperformance issues, durations from four milliseconds to one hundredmilliseconds may be utilized, with a typical duration of fortymilliseconds being sufficient to loosen the silt without causing unduemotion of the servo mechanism 110. However, longer or shorter durationsmay be set as required or desired.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference to the same extent asif each reference were individually and specifically indicated to beincorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. A method of preventing malfunction of acurrent-to-pressure converter (CPC), comprising the step of periodicallyimparting symmetrically-opposed movement of a control valve of the CPCto loosen and flush accumulated silt therefrom, wherein the step ofperiodically imparting symmetrically-opposed movement of a control valvecomprises the step of periodically introducing a small-amplitudesymmetrically-opposed impulse to a controller that actuates a hydrauliccontrol shaft of a three-way rotary valve, wherein the step ofperiodically introducing a small-amplitude symmetrically-opposed impulseto a controller comprises periodically introducing a small-amplitudesymmetrically-opposed impulse to a digital controller.
 2. The method ofpreventing malfunction according to claim 1, further comprising the stepof adjusting a period of the small-amplitude symmetrically-opposedimpulse.
 3. The method of preventing malfunction according to claim 1,further comprising the steps of: detecting a drive current levelrequired to actuate the hydraulic control shaft; increasing a frequencyat which the small-amplitude symmetrically-opposed impulse is introducedto the controller if the drive current is detected at a level thatindicates accumulated silt; and decreasing the frequency at which thesmall-amplitude symmetrically-opposed impulse is introduced to thecontroller if the drive current is detected at a level that does notindicate accumulated silt.
 4. The method of preventing malfunctionaccording to claim 1, further comprising the step of configuring thecontroller such that a user can set the period of the small-amplitudesymmetrically-opposed impulse.
 5. The method of preventing malfunctionaccording to claim 1, wherein the step of periodically introducing asmall-amplitude symmetrically-opposed impulse to a controller thatactuates a hydraulic control shaft of a three-way rotary valve comprisesperiodically introducing a small-amplitude symmetrically-opposed impulseto a controller that actuates a hydraulic control shaft via a rotaryactuator coupled to the three-way rotary valve.
 6. The method ofpreventing malfunction according to claim 5, wherein the rotary actuatoris a limited angle torque rotary actuator.
 7. The method of preventingmalfunction according to claim 1, further comprising the step ofdiagnosing input signals to the controller to determine reasonablenessof the input signal with respect to their use in controlling a positionof the 3-way rotary valve.
 8. The method of preventing malfunctionaccording to claim 1, wherein the step of periodically introducing asmall-amplitude symmetrically-opposed impulse to a controller comprisesperiodically introducing a symmetrically-balanced impulse so as not toperturb components controlled by the CPC.
 9. The method of claim 1,wherein small-amplitude symmetrically-opposed impulse has an impulseamplitude of 5% or less.
 10. The method of claim 9, wherein thesmall-amplitude symmetrically-opposed impulse has an impulse amplitudeof approximately 1%.
 11. The method of claim 1, wherein thesmall-amplitude symmetrically-opposed impulse has an impulse duration ofbetween 4 milliseconds to one hundred milliseconds.
 12. The method ofclaim 11, wherein the small-amplitude symmetrically-opposed impulse hasan impulse duration of approximately 40 milliseconds.